Offshore platforms or floating structures have been widely used as bases for exploitation of submarine oil field and undersea mineral resources.
There are two types of floating structures: one is the fixed type having a platform on supporting legs driven into the sea bottom, while the other is so-called tension leg type in which a platform having buoyancy is moored at a constant sea area by means of anchor ropes.
However, the fixed type floating structure is short for modern exploitation at deep sea areas, because it necessitates a large supporting structure, and the tension leg type structure is being moved into limelight.
FIG. 1 shows a construction of typical conventional tension leg type floating structure.
A working platform 1 is supported by a plurality of columns having buoyancy. Trusses 3 are provided between the columns and between the platform and columns, for tightly fastening them one another. The columns are connected to anchors 5 rigidly fixed to the sea bottom, through tension wires 4, so that the platform and the columns are maintained in half-submerged state against the buoyancy. In this type of structure, however, excessively large force is applied to each tension wire 4, when the platform rolls and pitches, presenting a fear of breakdown of the tension wires 4 which may result in downfall of the platform.
In addition, it is quite difficult to maintain the constant point at which the floating structure is to be moored, against horizontal forces applied thereto, e.g. forces by the tide, wave and wind.
In order to overcome these problems, a structure as shown in FIG. 2 has been proposed, in which a platform 1 and a bottle hull 7 having a large buoyancy are connected by means of columns 6 of small diameter and trusses 3. The structure is installed in such a manner tension wires 8 are stretched between the bottle hull 7 and sea-bottom anchors 5 in a diverging manner so as to maintain the bottle hull in the submerged state. This arrangement is effective to restrain the lateral displacement of the structure such as horizontal movement or rotation, and the influence of the wind and wave is diminished due to the reduced diameter of the columns 6.
However, even this improvement arrangement could not avoid the fear of breakage of the tension wires 8 which are always in tightened condition, due to an extraordinary force which may be applied to the platform 1, thus holding the possibility of danger of downfall of the platform.